In general, prostheses for replacing defective natural joints between bony shafts are well known. One example of a total joint replacement for the metatarsal phalangeal joint of the foot employs a collared hinge with opposing integral stems, all formed of silicone. To install such a device, an incision is made and the metatarsal and phalangeal bones exposed and resected. Axial channels are formed in the intramedullary canals of the proximal phalanx and metatarsal. The stems are then disposed in the channels, the collars seated against the ends of the bones, and the incision closed. An example of such a silicone hinge replacement is the Sutter Hinged Great Toe Joint Implant, marketed by Sutter Biomedical, Inc.
Non-constrained total joint replacement systems are also known. In the context of the metatarsophalangeal joint, such systems typically include separate metatarsal and phalangeal components configured to be disposed on the ends of the respective resected bony shafts and maintained in place by the soft tissue surrounding the bones.
One type of such non-constrained system employs a metatarsal component including a cap with a rearwardly projecting stem. The stem is received in a channel formed in the intramedullary canal of the metatarsal, and the cap, in effect, covers the end of the metatarsal head, wrapping around the sides of the metatarsal. Resection of the metatarsal in a plurality of planes (i.e., faceting themetatarsal head) is typically required for disposition of the metatarsal cap. The phalangeal component comprises a disc-shaped base with a curved bearing surface and a rearwardly projecting stem. The metatarsal component is typically formed of biocompatible metal, and the phalangeal portion formed of polyethylene or of polyethylene backed with metal.
Other non-constrained systems employ a metatarsal component comprising a domed head with a rearwardly projecting rectangular cross-section stem. The head includes a convex, part-spherical bearing surface extending to the periphery of the metatarsal component and defining the maximum transverse dimension of the metatarsal component. No portion of the metatarsal component extends transversely beyond the bearing surface. The phalangeal component includes a concave part-spherical bearing surface and a rearwardly projecting stem of generally rectangular cross-section. The radii of the respective bearing surfaces are equal. However, the surface area of the convex bearing surface on the metatarsal component is noticeably larger than the surface area of the concave bearing surface on the phalangeal component. The metatarsal component is formed of a substantially physiologically inert metal, such as orthochrome, and the phalangeal component formed of polyethylene. When implanted, the stems of the metatarsal and phalangeal components are received in respective channels formed in the medullary channels of the bones.
Such prior art devices, when implanted, dispose the convex bearing surface immediately and contiguously adjacent to the bone. The present inventors have found that this tends to limit the range of permitted motion in the joint, and renders the system susceptible to bony overgrowth. Such bony overgrowth tends to further limit the permissible range of motion in the joint. Further, various of such prior art systems employ a metatarsal component having a concave part-spherical rear surface. The metatarsal head is craterized to form a recess to receive the metatarsal component so that the edge of the bearing surface abuts metatarsal bone. Such an arrangement is not only particularly susceptible to bony overgrowth, but tends to initiate a periosteal reaction that promotes bony overgrowth. In addition, during extremes of flexion and extension, the phalangeal component tends to ride off of the metatarsal component onto part of the actual articular surface of the metatarsal head. The polyethylene component thus tends to articulate with already deceased cartilage, often exacerbating a diseased condition and breaking down the actual articular surface of the metatarsal head in the vicinity of the metatarsal component.
Examples of prior art non-constrained systems are the "Total Toe System" marketed by Biomet, Inc. and the system described in U.S. Pat. No. 4,156,296 issued May 29, 1979 to Johnson, et al.